Inserting an image into a pre-existing background group?

[Richmond]

I have a background group that spreads itself
over something like 50 cards; however, I am
beginning to have serious reservations about
the aesthetics of some of the images I use as
faux-buttons and should like to replace them.

My problem is two-fold:

1. Insert an image into a pre-existing
     background group so it shows up\
     across the 50 pages.

2. Remove an image in a similar situation.

sincerely, Richmond.